Narrowneck renewal deemed a success
The construction of Palm Beach Artificial Reef on the Gold Coast is due to be completed in October this year. If the project stays on schedule the artificial reef will be completed exactly twenty years after Narrowneck, the first artificial reef to be built on the Coast.
Unlike Palm Beach reef which is made of stone, Narrowneck was constructed using sand-filled geotextile bags. In the intervening years those bags shifted, settled into the sand, or split, often due to anchor strike, hence Gold Coast City Council began remedial work on the reef in late 2017, completing it in mid-2018.
The new work is now a year old, so the results can be assessed, and it can also be viewed against the almost-complete Palm Beach Reef.
Both Narrowneck and Palm Beach reefs are designed primarily to halt erosion - surfing amenity is a secondary consideration. There's only 100 metres of land separating the Nerang River and the ocean so the purpose of Narrowneck reef is to protect the isthmus by mitigating wave energy offshore and allowing a salient (sand build up) to grow on the beach.
Early observations of Narrowneck report favourable sand build up with a "distinct" and "consistent" salient of between 30 and 50 metres. These observations continued up until 2010. However, Google Earth's historical imagery shows no salient ever appearing shoreward from the reef.
In contrast to early reports, a City spokseperson recently told Swellnet that a salient was never expected to accrete at Narrowneck, rather the sand build up was supposed to occur offshore.
"Previous monitoring and analysis of sand volumes since construction of the artificial reef has shown that the reef promotes an increase of sand volumes in the offshore sand bars, rather than on the upper beach."
The reef renewal at Narrowneck was not intended to change this situation, "but to increase the volume of sand available within the sandbar system."
A year on, and with the City continually monitoring the sand flow, the City considers the project a success.
Anecdotally, the new geotextile bags at Narrowneck have settled with the reef becoming deeper and breaking less often, however those reports don't stack up against testing. "The City has surveyed the Narrowneck artificial reef regularly following the renewal. The survey data does not show any settlement of the Geotextile Sand Containers."
A similar discrepancy exists in how often the reef is being surfed with locals reporting few sessions, plus Swellnet's own Narrowneck camera showing surfers seldom surf the reef. These observations aren't supported by the City's own monitoring which has shown "a significant increase in the frequency of wave breaking on the reef", adding that "the enhanced influence of the reef on incoming waves, provides additional opportunities for surf amenity on the reef and also on the sand banks inshore and adjacent to the reef structure."
Lastly, despite the Palm Beach artificial reef using quarried stone, the City has no plans to switch materials at Narrowneck. All future renewal work will "likely to be undertaken using Geotextile Sand Containers consistent with the reef’s design."